The present invention relates to a molded plastic package for encapsulating a semiconductor device. More particularly, a heat slug is at least partially embedded in a molding resin to enhance the dissipation of heat from the device.
Molded plastic electronic packages provide environmental protection to integrated circuit devices. Packages such as the PQFP (plastic quad flat package) and PLCC (plastic leaded chip carrier) protect an encapsulated device from contaminants such as moisture and from mechanical shock.
One molded plastic package is illustrated in U.S. Pat. No. 4,707,724 to Suzuki et al, which is incorporated by reference in its entirety herein. The Suzuki et al. package has a leadframe with a centrally positioned die attach pad. A semiconductor device is bonded to the pad and electrically interconnected to the inner ends of the leadframe. A polymer molding resin encapsulates the device, die attach pad and inner lead ends.
One disadvantage with molded plastic packages is poor thermal dissipation. During operation, the semiconductor device generates heat which must be removed to maintain the operating integrity of the device. Some heat is dissipated through the bonding wires and leadframe, the remainder is absorbed into the molding resin. The molding resin is a poor thermal conductor so the device temperature increases. To prevent the device from overheating, the power provided to the device must be limited.
One way to improve the dissipation of heat from a semiconductor device is to incorporate a heat slug into the molded plastic package. The heat slug provides an enhanced path for thermal dissipation. As a result, more power may be provided to the semiconductor device without a resultant excessive increase in device temperature.
Heat spreaders and heat slugs are both thermally conductive structures embedded in a molding resin to enhance the dissipation of heat from a semiconductor device. Heat spreaders are completely encapsulated within the molding resin while heat slugs have at least one surface exposed to the outside environment. While the present invention will be described particularly in terms of a heat slug, all embodiments contained herein are equally applicable to heat spreaders and the term "thermal dissipator" encompasses heat slugs, heat spreaders and related structures.
The heat slug is usually nickel plated copper and will either be in direct contact with the semiconductor device or separated from the device by molding resin.
The heat slug is distinguished from a die attach pad. The die attach pad is formed from the same material as the leadframe and is the same thickness as the leads, generally between 0.13 millimeter and 0.51 mm (0.005-0.020 inch). The thin die attach pad has a limited heat capacity and its effectiveness in removing heat from a device is limited by the thermal conductivity of the molding resin. Heat slugs are not unitary with the leadframe and are, usually, thicker than the leadframe. This extra thickness enhances the thermal capacity of a heat slug increasing the capacity of the heat slug to remove heat from an integrated circuit device.
Copper heat slugs provide the molded plastic package with improved dissipation capability and are low cost. However, during solder plating of the leads, the solder wets the heat slug. This excess solder reflows during soldering of the leads to a printed circuit board and can cause an electrical short circuit between leads or between the heat slug and circuit traces on the printed circuit board.
Other disadvantages with copper heat slugs include:
The electrical conductivity of the heat slug does not provide electrical insulation between an electronic device and external circuitry. PA1 The weight of copper (density=8.93 g/cm.sup.3) increases the weight of a molded plastic package. PA1 The coefficient of thermal expansion of copper is about three times that of silicon. Direct bonding of a silicon based integrated circuit device to a copper heat slug induces strain in the device. During temperature cycling this strain may cause silicon fracture. PA1 The interface between the heat slug and a polymer molding resin is a site for the ingress of moisture. The moisture expands due to temperature increases and the package base can swell or fracture (the "popcorn effect"). PA1 The copper and nickel plate do not match the, typically, black color of most molding resins. The molding resin absorbs infrared (I.R.) radiation during I.R. solder reflow at a different rate than the metallic surfaces. The resultant localized heating can induce strains in the package during soldering.
Other than copper, heat slugs are formed from clad metals having a copper or aluminum component as disclosed in U.S. Pat. No. 5,015,803 to Mahulikar et al, which is incorporated in its entirety herein by reference.
One method to improve the adhesion of electronic package components to a molding resin is to form a plurality of grooves to mechanically lock the resin as disclosed in U.S. Pat. No. 4,589,010 to Tateno et al. Alternatively, the component is coated with a material having better adhesion to the molding resin as disclosed in U.S. Pat. No. 4,888,449 to Crane et al.
These approaches do not reduce the weight of the package and do not maximize the transfer of heat from a semiconductor device to a surface of the package.